The invention relates to packing structures between a shaft and a housing, and specifically to a packing structure for sealing between a valve stem and valve housing at both normal low temperature operating conditions and also at above normal high temperature conditions.
Valves utilized in pipelines that carry petroleum products are susceptible to exposure to abnormally high temperatures in the event of a fire in the pipeline. It is essential to effective operation that the valve, and in particular the seal between the valve stem and valve body, not leak under such abnormal operating conditions. Of course, the stem packing must also effectively seal at normal operating temperatures.
A variety of arrangements have been used in the past to provide for sealing between the valve stem and valve body at high temperatures. U.S. Pat. No. 3,788,600 prevents venting of the valve chamber to the atmosphere when there is an excessive pressure build-up and U.S. Pat. No. 4,082,105 provides for a secondary stem seal if the primary valve stem packing is destroyed by heat. In this arrangement, the secondary stem seat is activated at high temperatures and bites into the valve stem to impose a severe frictional drag.
Various other arrangements which have been utilized to provide high temperature sealing properties are exemplified by the following U.S. patents:
______________________________________ U.S. Pat. No. Inventor Issue Date ______________________________________ 2,647,721 Volpin August 4, 1953 3,177,887 Priese April 13, 1965 3,630,483 Camalizo Dec. 28, 1971 3,990,465 Allen Nov. 9, 1976 4,006,881 Guillard Feb. 8, 1977 4,090,719 Simanskie et al May 23, 1978 4,160,551 Nixon et al July 10, 1978 4,214,600 Williams, Jr. et al July 29, 1980 ______________________________________
In addition, several U.S. patents disclose the use of two sets of packings separated by a lantern ring or the like which is exposed to an external source of fluid lubricant or other injectible substance. U.S. patents that are exemplary of this type of structure are U.S. Pat. No. 2,883,214 issued on Apr. 21, 1959 to Perlaki, U.S. Pat. No. 3,096,070 issued on July 2, 1963 to Wolfensperger, and U.S. Pat. No. 3,907,307 issued on Sept. 23, 1975 to Mauer et al.
However, the prior art does not disclose the use of a low temperature sealing assembly in combination with a high temperature sealing assembly which are separated from one another in a packing chamber by a spacer ring or the like and which together are effective in providing a seal under both relatively low temperature are high temperature conditions and without the imposition of severe frictional drag as does the valve stem packing structure of this invention.